Rechargeable electrochemical cells such as batteries, have been known for over one hundred years. Many materials are known to be particularly useful in the fabrication of rechargeable electrochemical cells. Examples of these materials include nickel hydroxide, cadmium, lead, zinc, lithium and others. For example, zinc is a desirable negative electrode material as it is light in weight and has a theoretical capacity of 0.82 Ah/g. The reversible potential of zinc is high and it is relatively inexpensive. The hydrogen overpotential in zinc is high making it stable and efficient. The high exchange current of the zinc/zincate reaction results in high reversibility and high energy efficiency. Due to these advantages, many commercially available primary batteries use zinc as the negative electrode.
Despite these advantages, one serious weakness exists and is shared with lithium. This weakness is the short cycle life attributable to zinc. Short cycle life is due principally to one of two common failure mechanisms, which are: (1) dendritic formation on the zinc surface, leading to internal shorts as these dendrites grow through the separator and reach the other electrode; and (2) shape change in the zinc electrode leading to decreased capacity. The precise reason for dendritic growth of zinc is not fully understood, though workers in the field have hypothesized that small changes in local current result in a large difference in the amount of zinc deposited at a particular site. Due to these weaknesses, zinc has not become the negative electrode in a rechargeable battery.
Zinc ions dissolve in strong alkaline solution. The high ionic solubility together with the fact that the zinc deposition reaction is fast, makes zinc unstable and sensitive to the current density distribution during charge. Hence, small changes in local currents tend to result in substantial differences in the amount of zinc deposited at a particular site. Any non uniformity on the electrode surface would produce accelerated growth at the tip where the current density is the highest. As the tip grows into the bulk of the solution, higher Zn.sup.+2 concentration increases the growth rate and causes the dendrites to grow faster still.
Referring now to FIG. 1, there is illustrated therein a schematic representation of an electrochemical cell (10) according to the prior art. The cell (10) includes a positive electrode (20), a negative electrode (30) and a separator (40) disposed therebetween. Disposed on the side of the positive electrode (20) and negative electrode (30) opposite separator (40) is a layer of a current collecting material (50, 52) respectively. The current collector material may be fabricated of any of a number of different materials as are known in the art. With respect to the positive electrode (20), the negative electrode (30) and the separator material (40), each is fabricated of materials as are conventionally known in the art. For example, the negative electrode may be a cadmium electrode, a metal hydride electrode, a zinc electrode, a lithium electrode, and combinations thereof. The positive electrode may be a NiOOH electrode as from conventional nickel cadmium batteries, or other conventional positive electrodes such as carbon electrodes, graphite electrodes, and combinations thereof.
Workers in the field have tried numerous different ways to solve the problem of dendrite growth in conventional prior art cells as illustrated in FIG. 1. For example, the use of battery separators having very small openings, i.e., less than .about.300 angstroms, to block zinc dendrites from growing through the separator. Examples of these types of separator materials include cellophane, microporous polyethylene, and other polymeric materials. Unfortunately, internal cell resistance increases as pore size decreases. The power capacity of the battery decreases, though the battery cycle life has been extended. Another approach introduces vibration at the electrode to minimize non-uniformity in the deposition of zinc. Reported cycle life was extended to over 1,000 cycles, though the design could only be easily applied to flooded electrolyte cells.
Liquid membranes have been applied to try to reduce the dendrite shorting problem. For example, porous polymeric membrane with the pores filled with ion selective liquids which conduct hydroxide but reject Zn.sup.+2 ions have been attempted. Once again, however, internal resistance of the cell increases and thus power capacity of the battery decreases. Moreover, cycle life improvement was not dramatic in that only about 100 cycles have been reported.
A final approach has been to deposit a layer of metal on the microporous membrane. Such nickel coated membranes were used to reduce dendritic growth. The nickel layer is not connected to either electrode so that the electrochemical potential of nickel is more anodic than the zinc electrode during charge. As the zinc grows through the membrane, the tips will be oxidized and thus dissolved upon contact with the nickel layer. Cells with this type of membrane were found to increase cycle life of nickel zinc batteries from 85 cycles to approximately 425 cycles. An example of this approach is illustrated in U.S. Pat. No. 4,298,666 to Taskier.
Accordingly, there exists an economical, easy to implement approach for eliminating or reducing the results of dendritic growth in electrochemical cells, particularly those in which dendrite growth is dramatic. Examples of materials in which dendrite growth is a substantial problem include zinc and lithium batteries.